The present invention relates to a semiconductor laser device which can control the transverse mode of laser oscillation or has a transverse mode filter, and a method of fabricating the above device.
A method of fabricating a semiconductor laser device with a buried heterostructure, in such a manner that a laser active layer formed of a super lattice is partially converted into a mixed crystal by the impurity diffusion induced disordering, to obtain a large energy gap and a high refractive index in the mixed crystal region, is described in an article by T. Fukuzawa et al. (Appl. Phys. Lett., Vol. 45, 1984, page 1). In this method, crystal growth is carried out only once to form a semiconductor film indispensable for the laser device. That is, the method is suited for mass production of the laser device. However, the buried heterostructure formed by this method is of an index guiding type which generates a single transverse mode in the whole region of laser cavity, and thus a single longitudinal mode is generated Accordingly, the above method is not suited for forming a semiconductor laser device which is required to emit a laser beam of low optical feedback noise, such as a semiconductor laser diode for a compact disc.
Further, a semiconductor laser device which generates a multi longitudinal mode and in which proton implantation is used for current confinement, is described in an article by T. Mamine et al. (J. Appl. Phys., Vol. 54, 1983, page 4302). This laser, however, is of a gain guiding type, and hence a laser beam emitted from the laser device is large in astigmatism. Accordingly, in order to focus the laser beam on a very small spot, it is necessary to use an additional lens such as a cylindrical lens.
Further, a semiconductor laser device has been known in which an optical guide layer formed of a super lattice and disposed near a laser active layer is partially converted into a mixed crystal by impurity induced disordering, to form a stripe-shaped optical waveguide (refer to an article described by Kuroda et al. on pages L548 to L550 of the Japanese Journal of the Applied Physics., vol. 24, 1985). Now, the above semiconductor laser device will be explained below, by reference to FIG. 1. A p-GaAlAs layer 102 (serving as a cladding layer), a laser active layer 103, a super lattice layer 104 (serving as an optical guide layer), an n-type cladding layer 106 and an n-type cap layer 107 are successively grown to form a laser crystal on a P-GaAs substrate 101. A zinc-diffused region 111 is formed so that the optical guide layer 104 is partially converted into a mixed crystal region 105. The refractive index of the remaining super lattice region 104 is greater than that of the mixed crystal region 105, and hence the traverse mode of laser oscillation is confined in the super lattice region 104. In this laser device, however, the refractive index of the super lattice region 104 becomes smaller than that of the mixed crystal region 105, depending upon the amount of injected carrier, and hence it is impossible to obtain a stable transverse mode. Further, the super lattice layer is partially converted into the mixed crystal region by an electrically active impurity, and hence it is impossible to change the width of a stripe for index guiding, independently of the width of a stripe for current confinement.
Unlike an index guiding type semiconductor laser device a gain guiding type semiconductor laser device produces laser oscillation of multi longitudinal mode and emits a laser beam having a large amount of noise component. However, the noise generated when the laser beam is reflected back from a body, is low, that is, the laser beam is low in optical feedback noise, and hence the gain guiding type semiconductor laser device is suitable for use in a device which receives a large amount of reflected light, such as the pickup of a compact disc player. In the gain guiding type semiconductor laser device, however, the transverse mode of laser oscillation is unstable, and the far field pattern of the laser beam has a twin peak on the basis of a reduction in light intensity at a central portion of the laser beam due to the spatial hole burning. Thus, it is impossible to focus the laser beam on a single point.
Further, a semiconductor laser device has been known in which highly resistive regions formed by proton implantation are used for current confinement, and the gain varies along the lengthwise direction of laser cavity to solve the above problem of the gain guiding type semiconductor laser (refer to an article described by T. Mamine et al. on page 4302 of the J. Appl. Phys., Vol. 54, 1983). The formation of a highly resistive region by proton implantation has a drawback that the performance of the laser device is deteriorated because the highly resistive region is formed on the basis of defects produced by proton implantation. Further, the highly resistive region formed by proton implantation is extinguished by the annealing process at a temperature of 400.degree. to 500.degree. C. Accordingly, it is difficult to form a negative electrode which necessitates the alloying at temperatures higher than 400.degree. C. In other words, it is necessary for a region used for current confinement to have no defect or few defects and to withstand a high temperature annealing process.
As has been already mentioned, a method of fabricating a semiconductor laser device with a buried heterostructure, in such a manner that a laser active layer formed of a multi quantum well (namely, a super lattice) is partially converted into a mixed crystal by impurity induced disordering, is described in the abovereferred article by Fukuzawa et al. (Appl. Phys. Lett., Vol. 45, 1984, page 1). In this method, however, the width of an electrode stripe is inevitably smaller than the region. For example, it is necessary to form a stripe having a width of 1 .mu.m over a stripe having a width of 1.5 .mu.m by the photo-lithographic method. Accordingly, it is not easy to fabricate the semiconductor laser device.